It is desirable in an aircraft fuel tank system, to have an overpressure protector to relieve excess pressure to prevent the pressure from reaching a dangerous level.
In a fuel tank system, high pressure differentials may occur when, for example, the aircraft is being refuelled (resulting in high pressure in the tank relative to the atmosphere), and the fuel venting system between individual tanks becomes blocked (resulting in a large pressure differential across the fuel tanks).
It is known to fit an overpressure protector in the form of an overpressure valve. The overpressure valve is typically in the form of a poppet valve or the like and is adapted to move from a closed state to an open state in the event of an excessive pressure differential. Another known type of overpressure protector is a frangible disc. The frangible disc may be arranged to adopt an open state by failing (rupturing) when the pressure differential exceeds a certain level.
Where an overpressure protector is used for tank-to-tank pressure relief, it is often difficult, if not impossible, to tell from outside the fuel tanks whether the overpressure protector is in the open or closed state, or whether the protector has recently operated between those two states. It may therefore be necessary to regularly access the interior of the fuel tanks to check the state of the overpressure protector. Such checking may be expensive and/or time consuming.
A significant time period may elapse between an event causing the movement of the overpressure protector to the open state, and the act of checking the overpressure protector (by accessing the interior of the fuel tanks). Fault detection may therefore be delayed.
Overpressure valves tend to have a number of advantageous features over some other types of overpressure protector such as frangible discs (for example, frangible discs are limited to once-only operation). However, overpressure valves may be susceptible to jamming. In the event the overpressure valve jams, excessive pressure can build up leading to catastrophic failure of the system or component(s) associated with the overpressure valve. It may therefore be necessary to fit a different overpressure protector, such as a frangible disc (either instead of, or additional to, the overpressure valve) in order to provide a more reliable and/or a backup means of pressure relief. Whilst the different overpressure protector may be reliable, it will not necessarily be able to provide many of the advantages associated with an overpressure valve. The different overpressure protector may also have its own associated disadvantages, be expensive to fit and/or can add undesirable weight to the aircraft (for example due to the associated piping/venting systems required).
Overpressure valves tend to need to be manually reset from the open state to the closed state after use. Resetting the valve may involve accessing the interior of a fuel tank, or removing significant aircraft structure to access the overpressure valve. Thus, it can be an expensive and/or time consuming task to reset the valve.